Studies are in progress to determine the biochemical events necessary to ensure segregation of newly synthesized secretory proteins in the cisternae of the RER. Secretory proteins are believed to be synthesized as precursors containing an amino terminal peptide extension called the transport peptide which signals attachment of the mRNA - 80S - nascent polypeptide chain complex to the microsomal membrane. Cell biological experiments investigate reconstitution of rough microsomes using purified components (isolated polyA plus mRNA, microsomal membranes, and translation components). Protein sequencing studies are carried out using radioactive proteins synthesized in vitro.